Bissetii is tough stuff and is supposed to be hardy to -15F (at least in the ground). I wouldn't prune it back until mid-summer. Sometimes I've had winter damage and still got leaves on badly damaged culms, and it won't hurt or help the plant to prune back now. Just treat it as usual, give it some fertilizer, and make your pruning decisions in a couple of months..

mine is looking similarly bad. i have it in a large insulated box in the yard. looked ok all winter but now all white and dead looking. the culms are green at the bottom so i think it'll be ok. last year it looked ragged too but then the leaves fell off and came back in nice and new. i'm planning to sit tight and hope they come back ok.